The symbol is not used outside of the file, so mark it static.

Fixes the following warning:

./drivers/gpu/drm/nouveau/nvkm/engine/disp/gv100.c:591:1: warning:
symbol 'gv100_disp_core_mthd_base' was not declared. Should it be static?
Signed-off-by: ruanjinjie <ruanjinjie@huawei.com>
Reviewed-by: Karol Herbst <kherbst@redhat.com>
Signed-off-by: Karol Herbst <kherbst@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220924073957.4140388-1-ruanjinjie@huawei.com